1. Field of the Invention
The present invention pertains to an A/D (analog/digital) converter and more particularly to an integrated, single-ended input A/D converter that has a high-speed and low current consumption for generating a differential output.
2. Description of the Related Art
It is very common for electronic equipment to require analog signals to be converted into digital signals. A significant example of this type of equipment is provided by digital cellular telephones. The requirements of low consumption and high performance associated with these applications create particularly exacting conditions for the design of the integrated circuits that constitute the electronic system of the equipment.
A type of A/D converter particularly advantageous for employment in these electronic systems is the one that makes use of capacitors weighted in binary code with associated switches that are controlled by a logic circuit in accordance with a technique known as SAR (Successive Approximation Register).
When designing complex integrated circuits, it is generally sought to obtain a large dynamic range and a good immunity to noise originating both within and without the integrated circuit by realizing the part that processes the analog signal with structures of a completely differential type.
In certain cases, however, the analog signals that have to be converted into digital form are not of the differential type, that is to say, in the form of voltage variations of a sign opposite to a common reference potential, but are rather of an asymmetric or single-ended type, i.e. in the form of a single voltage variable with respect to a reference potential. In these cases the designer has two alternatives: one can either use A/D converters that function with single ended input signals or transform the signal into a differential signal and then apply it to an A/D converter that functions with differential input signals. In the former case one has to find means, if possible, for avoiding the negative effects of noise and the other intrinsic limitations associated with the processing of a single-ended signal, while in the latter case one has to accept a higher consumption and other undesired effects (distortion and offset) of the circuit that has to precede the converter.
With a view to obviating the drawbacks of both these alternatives, there has been proposed an A/D converter with single-ended input, as described in EP-A-1039642, that presents the advantages of a differential structure without requiring a circuit to transform the single-ended signal into a differential one. This known converter makes use of four capacitance arrays in place of the two arrays provided in a converter with a differential input, and from the constructional point of view this means that the integrated circuit comprising the converter will occupy a rather large area, while from the functional point of view it brings in its wake a considerable increase in electric power consumption and the need for having to use a high-sensitivity comparator to compensate the smaller voltage excursion at the comparator input. This implies a reduced operating speed or, alternately, the construction of a comparator providing a better performance and therefore inevitably a larger size and costlier energy consumption.